Engines, including diesel engines, are being designed to operate under lean conditions as a fuel economy measure. Such future engines are referred to as “lean burn engines.” That is, the ratio of air to fuel in the combustion mixtures supplied to such engines is maintained considerably above the stoichiometric ratio (e.g., at an air-to-fuel weight ratio of 18:1) so that the resulting exhaust gases are “lean,” i.e., the exhaust gases are relatively high in oxygen content. Although lean-burn engines provide advanced fuel economy, they have the disadvantage that conventional three-way catalytic converters (TWC) are not effective for reducing NOx emissions from such engines because of excessive oxygen in the exhaust. Attempts to overcome this problem have included the use of a NOx trap. The exhaust of such engines are treated with a catalyst/NOx sorbent which stores NOx during periods of lean (oxygen-rich) operation, and releases the stored NOx during the rich (fuel-rich) periods of operation. During periods of rich (or stoichiometric) operation, the catalyst component of the catalyst/NOx sorbent promotes the reduction of NOx to nitrogen by reaction of NOx (including NOx released from the NOx sorbent) with hydrocarbon (HC), carbon monoxide (CO), and/or hydrogen present in the exhaust.
Diesel engines provide better fuel economy than gasoline engines and normally operate 100% of the time under lean conditions, where the reduction of NOx is difficult due to the presence of excess oxygen. In this case, the catalyst/NOx sorbent is effective for storing NOx. After the NOx storage mode, a transient rich condition must be utilized to release/reduce the stored NOx to nitrogen.
In a reducing environment, a lean NOx trap (LNT) activates reactions by promoting a steam reforming reaction of hydrocarbons and a water gas shift (WGS) reaction to provide H2 as a reductant to abate NOx. The water gas shift reaction is a chemical reaction in which carbon monoxide reacts with water vapor to form carbon dioxide and hydrogen. The presence of ceria in an LNT catalyzes the WGS reaction, improving the LNT's resistance to SO2 deactivation and stabilizing the PGM.
The lean operating cycle is typically between 1 minute and 20 minutes and the rich operating cycle is typically short (1 to 10 seconds) to preserve as much fuel as possible. To enhance NOx conversion efficiency, the short and frequent regeneration is favored over long but less frequent regeneration.
The LNT catalyst operates under cyclic lean (trapping mode) and rich (regeneration mode) exhaust conditions during which the engine out NO is converted to N2 as shown in equations 1-6:Lean condition: 2NO+O2→2NO2  (1)(Trapping mode)4NO2+2MCO3+O2→2M(NO3)2+2CO2  (2)Rich condition: M(NO3)2+2 CO→MCO3+NO2+NO+CO2   (3)(Regeneration mode) NO2+CO→NO+CO2  (4)2NO+2CO→N2+2CO2  (5)2NO+2H2→N2+2H2O  (6)
In preparation for the emerging Euro 6 automotive exhaust emission catalyst market to meet increasingly stringent NOx emissions, diesel oxidation catalysts (DOC) for diesel passenger cars may be replaced with a close-coupled lean NOx trap with diesel oxidation functionality for engine displacements ranging from 1.2 to 2.5 L. In addition to managing NOx emissions from the vehicle, this change will require the LNTDOC to effectively oxidize engine-out hydrocarbon (HC) and carbon monoxide (CO) emissions. Specifically, this change requires that the LNT fulfill the de-NOx function of converting NOx to N2 while also taking on the dual role of a DOC to oxidize engine-out hydrocarbons (HC) and carbon monoxide (CO) (Equations 7 and 8) and to generate an exotherm for the regeneration of a catalyzed soot filter (CSF).HC and CO oxidation:CxHy+O2→CO2+H2O  (7)2CO+O2→2CO2  (8)
New legislation, such as the Diesel Euro 6b and Diesel Euro 6c legislation, requires a reduction in carbon dioxide (CO2) emissions. To comply with such legislation, engine calibrations for light duty diesel applications will have to be implemented to obtain a reduction in carbon dioxide (CO2) emissions. In practice, the reduction in CO2 emission will result in a lower temperature in front of the carbon monoxide (CO) and hydrocarbon (HC) oxidation catalysts during driving cycle in vehicles using such catalysts. For systems having a lean NOx trap (LT-LNT) composition that stores NOx before the SCR light-off to result in lower NOx emissions, the removal of the stored NOx and conversion to N2 at lower temperatures is a challenge.
In addition in some countries, new On-Board Diagnostic (OBD) regulations require that the Diesel Oxidation Catalysts (DOC) function (mainly HC conversion) in a DOC-SCR system to be monitored during driving. At present, there is no working DOC monitoring method available which can monitor the aging state of a DOC because there is no existing HC sensor. Therefore, it would be desirable to provide a DOC composition that can provide OBD capability.